The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for monitoring a process in a plasma processing system by measuring self-bias voltage.
In the processing of a substrate, e.g., a semiconductor substrate or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In an exemplary plasma process, a substrate is coated with a thin film of hardened emulsion (i.e., such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing components of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck or pedestal. Appropriate etchant source are then flowed into the chamber and struck to form a plasma to etch exposed areas of the substrate.
Referring now to FIG. 1, a simplified diagram of a capacitively coupled plasma processing system is shown. Generally, capacitively coupled plasma processing systems may be configured with a single or with two separate RF power sources. Source RF, generated by source RF generator 134, is commonly used to generate the plasma as well as control the plasma density via capacitively coupling. While bias RF, generated by bias RF generator 138, is commonly used to control the DC bias and the ion bombardment energy. Further coupled to source RF generator 134 and bias RF generator 138 is matching network 136, that attempts to match the impedance of the RF power sources to that of plasma 110. In addition, matching network 136 may also include a V/I probe (not shown) that can measure the voltage and impedance of a current transmitted to plasma 110, as well as the ability to modify a generated plasma frequency in order to better optimize the plasma to process conditions.
Generally, an appropriate set of gases is flowed into chamber 102 through an inlet in a top electrode 104 from gas distribution system 122. These plasma processing gases may be subsequently ionized to form a plasma 110, in order to process (e.g., etch or deposition) exposed areas of substrate 114, such as a semiconductor substrate or a glass pane, positioned with edge ring 115 on an electrostatic chuck 116, which also serves as an electrode
Commonly, a cooling system 140 is coupled to electrostatic chuck 116 in order to achieve thermal equilibrium once the plasma is ignited. The cooling system itself is usually comprised of a chiller that pumps a coolant through cavities in within the chuck, and helium gas pumped by pump 111 between the chuck and the substrate (e.g., backside He Flow). In addition to removing the generated heat, the helium gas also allows the cooling system to rapidly control heat dissipation. That is, increasing helium pressure subsequently also increases the heat transfer rate. Most plasma processing systems are also controlled by sophisticated computers comprising operating software programs. In a typical operating environment, manufacturing process parameters (e.g., voltage, gas flow mix, gas flow rate, pressure, etc.) are generally configured for a particular plasma processing system and a specific recipe.
In a common substrate manufacturing method, known as dual damascene, dielectric layers are electrically connected by a conductive plug filling a via hole. Generally, an opening is formed in a dielectric layer, usually lined with a TaN or TiN barrier, and then subsequently filled with a conductive material (e.g., aluminum (Al), copper (Cu), etc.) that allows electrical contact between two sets of conductive patterns. This establishes electrical contact between two active regions on the substrate, such as a source/drain region. Excess conductive material on the surface of the dielectric layer is typically removed by chemical mechanical polishing (CMP). A blanket layer of silicon nitride is then deposited to cap the copper.
However, in these and other plasma processes, it is often difficult to determine exactly when process conditions change beyond established parameters. In particular, as device dimensions shrink and more advanced low k materials are used, the requirements for substantially stable process conditions become even more stringent in order to maintain a uniform etch rate, improve yield, etc.
Contamination, in particular, tends to present a substantial problem. The degree of contamination is usually dependent on the specific plasma process (e.g., chemistry, power, and temperature) and the initial surface condition of chamber. Since fully removing deposits may be time consuming, a plasma processing system chamber is generally only substantially cleaned when the particle contamination levels reach unacceptable levels, when the plasma processing system must be opened to replace a consumable structure (e.g., edge ring, etc.), or as part of scheduled preventive maintenance (PM).
Likewise, hardware deterioration also tends to be problematic. As plasma chamber components are exposed to the plasma, they themselves may become damaged, altering mechanical and electrical characteristics, as well as producing contaminants. In fact, the cleaning process itself may damage the components, as with the electrostatic chuck (chuck) during waferless auto clean or (WAC).
Yet, there is generally no effective way to determine if a plasma process has moved outside of established parameters in-situ, without first initially processing and then subsequently testing partially manufacturing substrates. That is, after a batch of substrates has been processed, a sample substrate is removed from the batch and tested. If the test determines that the substrate does not meet the established specification, the entire batch of substrates may need to be destroyed.
One solution may be to create a simplified empirical model of the plasma processing system in order to sufficiently capture the behavior of the tool. However, creating an empirical model may be problematic. For example, a modified non-operational plasma chamber may be analyzed in order to extract parameters for the simplified empirical. In another technique, the individual components of a plasma processing system may be individually measured using a network analyzer.
However, even a loosely correlated (and hence weakly predictive) model is difficult to obtain since repetition of the plasma process itself may effect of the electrical characteristics of plasma processing system components. The creation of simplified empirical models may only be done infrequently, and only by trained personnel.
In view of the foregoing, there are desired methods and apparatus for monitoring a process in a plasma processing system by measuring self-bias voltage.